Acrylic-functional resins are widely employed in radiation curing as the principal film-forming materials due to their high cure rate and favorable combination of weathering, chemical resistance and mechanical properties. These resins usually range in molecular weight from about 500 to about 6,000 and encompass several distinct classes including the epoxy acrylates, urethane acrylates, polyester acrylates, and others.
Pruncal, et al., U.S. Pat. No. 3,874,906 teach a method of applying and curing a polyester-acrylate containing coating composition comprising adding N-vinylpyrrolidone to the composition and subsequently applying the composition to a substrate and subjecting it to actinic light to cure. Lorenz, et al., U.S. Pat. No. 4,129,709 disclose a coating composition comprising an oligomer produced by reacting polytetrahydrofuran with a diisocyanate, N-vinyl-2-pyrrolidone and an acrylic acid ester having a boiling point of a least 200.degree. C. at 760 mm Hg.
Priola. et al., U.S. Pat. No. 4,348,427 teach a method of coating surfaces by applying to the surface to be coated, a mixture composed of at least one compound from the following classes: epoxy-acrylate resins, polyester alpha,omega-acrylate resins, unsaturated polyester resins, or urethane-acrylate resins; plus at least one unsaturated compound of the amide, lactam, piperidone or urea classes, and subsequently irradiating the coated surface with radiation in the range of 200 to 400 nm.
In recent years concerns have increased about the toxicity or carcinogenic potential of the acrylic materials used in radcure systems, particularly those of low molecular weight. 2-Hydroxyethyl acrylate, which may be present as a residual in the urethane acrylates described above, is highly toxic by skin absorption and inhalation, making it undesirable as the source of unsaturation. Such concerns have stimulated interest in so-called "Non-Acrylate" radcure chemistries. So far, however, these systems have generally failed to match the cost/performance properties of acrylates.
Vinyl ether terminated resins have been suggested as alternatives to acrylated materials in that they have the advantages of lower toxicity and the ability to cure by both cationic and free radical mechanisms. Urethane versions are produced by reaction of a hydroxy vinyl ether and an isocyanate prepolymer in a method analogous to the urethane acrylates. Lapin, et al., U.S. Pat. No. 4,751,273 disclose vinyl ether terminated urethane resins prepared by reacting the product obtained by the addition of acetylene to an organic polyol with an isocyanate-containing compound at temperatures ranging from about ambient to 125.degree. C. However, such vinyl ether resins suffer from the disadvantages of high cost and inhibition of cure by atmospheric moisture or Lewis bases when polymerized cationically.
Free radical curing of vinyl ethers with acrylates or maleates has also been described, however, these systems require careful control of vinyl ether/acrylate or vinyl ether/maleate stoichiometry due to the particular reactivity ratios of the components. They may also suffer from inferior cure speed, high color, and higher viscosities than all-acrylate systems.
Barzynski, et al., in U.S. Pat. Nos. 4,205,139 and 4,424,314 disclose curable coating compositions which contain one or more olefinically unsaturated compounds with a molecular weight from 70 to 2,000 and one or more different compounds which are copolymerizable with the above compound. The second compound has a molecular weight from 110 to 2,000 and has at least 2 N-vinyl groups and at least one carbonyl group bonded to the nitrogen of at least one N-vinyl group.
P. Kuntz and H. Disselnkotter, Liebigs Ann. chem., 764, 69-93 (1972) report the synthesis and reactions of a number of vinyl amides, including N-vinylformamide (NVF). They teach that addition to isocyanates was successful only with NVF to yield N'-substituted N-vinyl-N-formylurea.